photoconversion of semiconducting photoelectrodes in photo-electrochemical (PEC) water-splitting, it is necessary to synthesize high-quality single-crystal thin films. [1-7] High-crystalline oxide thin films provide an ideal platform to enhance photoconversion efficiency using various strategies such as ferroelectric oxide domain engineering and interfacial engineering. [3-7] For example, the interlayer insertion of atomically sharp polar LaAlO 3 lowered the charge transfer resistance between the WO 3 photoanode and the Nb:SrTiO 3 substrate. [6] Furthermore, crystal facet engineering, which controls the preferred crystallographic orientation, has become an emerging approach to enhance the photocurrent or photostability in oxidebased photoelectrodes such as TiO 2 , BiVO 4 , and Cu 2 O thin films. [8-12] Therefore, the synthesis of single-crystal oxide thin film photoelectrodes is significantly important for the development of photoelectrodes toward efficient PEC water-splitting. While the investigation of single-crystal thin film photoanodes such as TiO 2 and BiVO 4 are relatively focused, [13-15] there are few studies for single-crystal thin film photocathodes such as copper-based oxides. One of the major challenges facing To develop strategies for efficient photo-electrochemical water-splitting, it is important to understand the fundamental properties of oxide photoelectrodes by synthesizing and investigating their single-crystal thin films. However, it is challenging to synthesize high-quality single-crystal thin films from copperbased oxide photoelectrodes due to the occurrence of significant defects such as copper or oxygen vacancies and grains. Here, the CuBi 2 O 4 (CBO) single-crystal thin film photocathode is achieved using a NiO template layer grown on singlecrystal SrTiO 3 (STO) (001) substrate via pulsed laser deposition. The NiO template layer plays a role as a buffer layer of large lattice mismatch between CBO and STO (001) substrate through domain-matching epitaxy, and forms a type-II band alignment with CBO, which prohibits the transfer of photogenerated electrons toward bottom electrode. The photocurrent densities of the CBO single-crystal thin film photocathode demonstrate −0.4 and −0.7 mA cm −2 at even 0 V RHE with no severe dark current under illumination in a 0.1 m potassium phosphate buffer solution without and with H 2 O 2 as an electron scavenger, respectively. The successful synthesis of high-quality CBO single-crystal thin film would be a cornerstone for the in-depth understanding of the fundamental properties of CBO toward efficient photo-electrochemical water-splitting.